Field of the Invention
The present invention relates to a toner that is used in recording methods that use, for example, an electrophotographic process.
Description of the Related Art
Image-forming apparatuses, e.g., copiers, printers, and so forth, have in recent years been subjected to increasing diversification with regard to their intended applications and their use environment, and along with this, higher speeds, higher image quality, and higher stability are being required. At the same time, copiers and printers are also undergoing device downsizing and advances in energy conservation.
Optimization of each of the electrophotographic process steps is critical for responding to the increases in image quality and greater energy conservation of recent years. With regard to the image quality in particular, optimization of the developing step, in which an electrostatic latent image is developed with toner to form a toner image, has been crucial. With regard to the energy conservation, the execution of a satisfactory fixing at low temperatures is crucial.
The use in toner of a crystalline polyester that induces the melt deformation of the toner particle by rapidly compatibilizing into the binder resin in the toner has been widely investigated in recent years as a means for improving the fixing performance (refer to Japanese Patent Application Laid-open No. 2013-137420, Japanese Patent Application Laid-open No. 2013-15673, and Japanese Patent Application Laid-open No. 2011-237801). At around its melting point, a crystalline polyester that has a strong effect on the low-temperature fixability readily compatabilizes into the binder resin and a rapid melt deformation by the toner during fixing is facilitated. Due to this, the low-temperature fixability of the toner is improved by the use of a crystalline polyester. In addition, the co-use of a wax can provide the toner with the ability to release from the fixing unit and thus can also be expected to provide additional improvements in the fixing performance.
However, since the crystalline polyester has the property of readily compatibilizing into the binder resin, the presence of the crystalline polyester at the toner particle surface is then facilitated and a lowering of the charging stability of the toner is readily induced. A lowering of the charging stability of the toner facilitates a lowering of the image density through a reduction in the developing performance. Moreover, storage in a severe environment of repetitive temperature increases and decreases (also referred to below as heat cycling) facilitates outmigration to the toner particle surface by the crystalline polyester compatibilized in the binder resin. As a result, the surface composition of the toner ends up fluctuating pre-versus-post-heat cycling and, for example, properties such as the fogging and so forth then undergo a substantial decline.
To respond to this problem, investigations have been carried out into lowering the amount of crystalline polyester that compatibilities into the binder resin. This lowering of the amount of compatibilization means achieving a state in which the crystalline polyester has a high degree of crystallinity. Investigations such as the following have been carried out in relation to toner production methods aimed at inducing the crystallization of the crystalline polyester. In accordance with Japanese Patent Application Laid-open No. 2010-145550, the degree of crystallinity of the crystalline polyester is enhanced through control of the cooling rate. In accordance with Japanese Patent Application Laid-open No. 2014-211632, the degree of crystallinity is enhanced by providing an annealing treatment step during cooling.
However, there is room for improvement with regard to Japanese Patent Application Laid-open No. 2010-145550 and Japanese Patent Application Laid-open No. 2014-211632 from the standpoint of reducing the charging stability caused by the presence of crystalline polyester at the toner particle surface and from the standpoint of the resistance to heat-cycling environments when the assumption is made of, for example, various logistics.
Moreover, when trying to focus on the fixing step from the perspective of the demand for higher image quality, a problem that shows up accompanying the diversification in the intended applications and the use environment is the problem of offset at the back end of a high print percentage image in high-temperature, high-humidity environments.
When, in the fixing step, the paper bearing the unfixed toner image is passed through the fixing unit (the transit region in particular is called the fixing nip herebelow), in general the toner is fixed to the paper by the application of heat and pressure.
The reason that offset is more severe with a high print percentage image than with a low print percentage image is thought likely to reside in the amount of heat applied to the toner layer. With higher print percentage images, the amount of heat from the fixing unit is dispersed into larger amounts of toner, and due to this a trend is set up of an increasing amount of toner that is inadequately melted. That is, a state is assumed in which the occurrence of fixing defects is facilitated.
Moreover, the amount of heat applied from the fixing nip part declines as the back end of the paper is approached, and due to this the occurrence of an unfavorable fixing performance at the back end of the paper is facilitated.
In particular, this offset phenomenon tends to become severe with paper that has been held in a high-temperature, high-humidity environment. This is hypothesized to likely be due to the following: when paper containing large amounts of moisture due to a holding period is passed through the fixing unit, water vapor is generated from the paper in the fixing nip part due to the heat received from the fixing unit and as a result the toner layer on the paper is forced toward the fixing film side.
That is, the appearance of the aforementioned offset phenomenon is facilitated when paper that has been held in a high-temperature, high-humidity environment is used under the circumstance that the occurrence of defective fixing at the back end of a high print percentage image is facilitated.
Improvements have been made, for example, the design of a low softening temperature, in order to improve the fixing performance of toner from the existing situation. However, with such a design, while the thermal melting behavior is improved in regions where heat is adequately applied, where the amount of heat applied is not adequate, for example, at the back end of a high print percentage image, the melting speed of the toner does not catch up and the suppression of back end offset with a high print percentage image has thus been quite problematic. In view of the preceding, there is demand for a toner that, even in a high-temperature, high-humidity environment, can suppress the occurrence of back end offset with a high print percentage image and that, even after exposure to a history of heat cycling, can provide a high-quality, fogging-inhibited image.